A rat revertant cell line (F-1) has been isolated, which is resistant to trans-formation by the mos oncogene, and can be used as a model for studying mechanisms of mos transformation and suppression of transformed phenotypes in vitro. Rat cells doubly transformed by mos (DTM) were generated by initially transforming immortalized rat fibroblasts with the ts Moloney murine sarcoma virus (Mo-MuSV ts110), and then transforming these cells with a wild-type Mo-MuSV at the non-permissive (non-transform- ing) temperature for the ts virus. Subsequently, following transfection of DTM cells with a retroviral construct containing the neo gene, under the control of the RAV-1 LTR, and selection of G418-resistant colonies, a transformation-suppressed revertant cell line (F-1) was isolated which exhibited a flat, non-transformed morphology and anchorage-dependent growth. Like the DTM parental cell line, F-1, revertants contained two integrated copies of the v-mos gene, expressed elevated levels of mos- specific RNA, and rescuable transforming viruses, indicating that the revertant phenotype observed in these cells did not result from loss or inactivation of v-mos or inhibition of its expression. The F-1 revertant line was sensitive to transformation by ras-H (Harvey sarcoma virus), but resistant to retransformation by mos (MSV124).